The trend in electronic equipment is towards higher clock frequencies and lower voltage levels. As a consequence, electronic apparatus become more and more susceptible to external electromagnetic interference and, on the other hand, they themselves produce more electromagnetic interference. The conductor layers of a printed circuit board form a waveguide which is capable of carrying high-frequency electromagnetic disturbances. For example, the power supply layer and the power supply ground plane can carry high-frequency electromagnetic disturbances to the power supply terminals of circuit components, causing the supply voltage of the circuit components to deviate from the desired value. When the value of the supply voltage differs too much from the desired value the circuit component will cease to function in the desired manner.
Supply voltage disturbances are generally suppressed using a capacitor connected to the power supply terminals of the circuit component, with the idea of providing a low-impedance bypass route for the spurious signals. A problem with capacitor-based noise suppression is the parasitic series inductance of the capacitor the effect of which increases as the frequency rises. Therefore, capacitor-based noise suppression usually no longer works in the desired manner when the noise appears at high frequencies.
An alternative solution for noise suppression is provided by a filter structure integrated in the printed circuit board, implementable with the electromagnetic band-gap (EBG) structure. The structure is designed to produce a stop-band in the frequency region where the spurious signals appear. A mushroom type filter structure comprises periodic conductive areas and feedthrough connections between the conductor layers of the printed circuit board which together with the conductor layers form a noise-suppressing filter. In a planar type filter structure a conductor layer in the printed circuit board comprises a periodic pattern with interconnected pattern elements. Each pattern element includes at least one high-impedance portion the inductance of which is dominant over the capacitance against a second conductor layer of the printed circuit board, and at least one low-impedance portion the capacitance of which against the second conductor layer is dominant over the inductance. The high-impedance portions are oblong strip conductors with a ratio of length and surface area such that the inductance seen by the longitudinal electric current in the strip conductor is dominant over the capacitance of the strip conductor. The low-impedance portion is a conductive region with an area/maximum diameter ratio so large that the capacitance is dominant over the inductance. In this document, the maximum diameter of a region refers to the greatest possible distance between two points belonging to the region. The high-impedance and low-impedance portions of the interconnected pattern elements form a filter capable of suppressing spurious signals coupled between the conductor layers. PCB-integrated electromagnetic band-gap filter structures can be implemented using a cascade structure in which various periodic patterns and/or various sub-regions of a mushroom type filter structure are connected one after another. The cascade structure allows the implementation of stop-bands of a desired width, for example. A limitation of the cascade structure is, however, that the characteristics achieved through the cascade are present only in conjunction with spurious signals the propagation direction of which is substantially the same as the direction where the various periodic patterns and/or various sub-regions of a mushroom type filter structure are cascaded.
An advantage of a planar type filter structure over a mushroom type filter structure is that in a planar type filter structure there is no need for inter-layer conductive regions and feedthrough connections which complicate the printed circuit board and increase manufacturing costs. A drawback of a planar type filter structure is that in addition to the electric current which represents noise, also the electric current which represents the useful signal or the transfer of power flows through the above-mentioned oblong strip conductors, whereby the resistance of the strip conductors limits the power transfer capacity of the filter structure. Considering the power transfer capacity of a filter structure, the oblong strip conductors should therefore be as short and wide as possible. However, the length/width ratio of the strip conductors must be sufficiently large for the stop-band of the filter structure to be wide enough. Publication US2008072204 discloses a solution with meandering striplines to achieve sufficient length for the striplines and, therefore, a stop-band of sufficient width. Making the strip conductors longer increases their resistance and, therefore, degrades the power transfer capacity of the filter structure. So, when designing the length of strip conductors one has to strike a compromise between the power transfer capacity of the filter structure and the width of the stop-band. However, finding a satisfactory compromise between the power transfer capacity and width of the stop-band may in some cases prove difficult.